To obtain hydrocarbons such as oil and gas, wellbores (also referred to as the boreholes) are drilled by rotating a drill bit attached at the end of a drilling assembly generally referred to as the “bottom hole assembly” (BHA) or the “drilling assembly.” A large portion of the current drilling activity involves drilling highly deviated and substantially horizontal wellbores to increase the hydrocarbon production and/or to withdraw additional hydrocarbons from the earth's formations. The wellbore path of such wells is carefully planned prior to drilling such wellbores utilizing seismic maps of the earth's subsurface and well data from previously drilled wellbores in the associated oil fields. Due to the very high cost of drilling such wellbores and the need to precisely place such wellbores in the reservoirs, it is essential to frequently determine the position and direction of the drilling assembly and thus the drill bit during drilling of the wellbores. Such information is utilized, among other things, to monitor and adjust the drilling direction of the wellbores. It should be noted that the terms “wellbore” and “borehole” are used interchangeably in the present document.
In the commonly used drilling assemblies, the directional package commonly includes a set of accelerometers and a set of magnetometers, which respectively measure the earth's gravity and magnetic fields. The drilling assembly is held stationary during the taking of the measurements from the accelerometers and the magnetometers. The toolface and the inclination angle are determined from the accelerometer measurements. The azimuth is then determined from the magnetometer measurements in conjunction with the tool face and inclination angle. As used herein, the term “toolface” means the orientation angle of the bent housing or sub in the borehole with respect to a reference such as high side of the borehole which indicates the direction in which the borehole will be curving. The inclination angle is the angle between the borehole axis and the vertical (direction of the gravity field). The azimuth is the angle between the horizontal projection of the borehole axis and a reference direction such as magnetic north or absolute north.
The earth's magnetic field varies from day to day, which causes corresponding changes in the magnetic azimuth. The varying magnetic azimuth compromises the accuracy of the position measurements when magnetometers are used. Additionally, it is not feasible to measure the earth's magnetic field in the presence of ferrous or ferromagnetic materials, such as casing and drill pipe. Gyroscopes measure the rate of the earth's rotation, which does not change with time nor are the gyroscopes adversely affected by the presence of ferrous materials. Thus, in the presence of ferrous materials the gyroscopic measurements can provide more accurate azimuth measurements than the magnetometer measurements.
U.S. Pat. No. 5,432,699 of Hache et al. discloses a method and apparatus measuring motion signals of gyroscopes in downhole instruments used to determine the heading of a borehole. Accelerometer and magnetometer data along three orthogonal axes of a measurement sub are used to obtain unit gravitational and magnetic vectors. The gyroscope measurements are used to correct the magnetic and gravity measurements made by the magnetometer and the accelerometer respectively. The calculations performed in the correction process by this, and other prior art optimization schemes based upon least squares methods, are valid when the measurements are corrupted by random additive noise. As would be known to those versed in the art, in the presence of systematic measurement errors, such least-squares optimization methods are unreliable.
Commercially available gyroscopes contain systematic errors or biases that can severely deteriorate accuracy of a gyroscope's measurements and thus the azimuth. Gyroscopes have been utilized in wireline survey applications but have not found commercial acceptance in the measurement-while-drilling (MWD) tools used in bottomhole assemblies.
In wireline applications, the survey tool is conveyed into the wellbore after the wellbore has been drilled, in contrast to the MWD tools wherein the measurements are made during the drilling of the wellbores. Wireline methods are not practical in determining the drilling assembly position and direction during the drilling of the wellbores. In wireline applications, the gyroscopes are used either in a continuous mode or at discrete survey intervals. Wireline survey methods often make it unnecessary to employ techniques to compensate for the present-value of the gyroscope biases. In wireline applications, the gyroscope can be powered-up at the surface and allowed to stabilize (thermally and dynamically) for a relatively long time period. Typically a warm-up period of ten (10) minutes or more is taken. The power to the gyroscope is continuously applied from the beginning at the surface, through the actual wellbore survey and through the final check of the survey tool at the surface at the end of the survey. Therefore, reference alignments can be made at the surface prior to commencing the wellbore survey to adjust the drift in a gimbaled gyroscope or verify the alignment accuracy of a north-seeking gyroscope. The initial independent reference can then be used at the end of the wireline survey. Any bias in the gyroscope in a wireline tool can be measured at the surface by taking the difference in the alignments at the beginning and the end of the survey runs. Furthermore, the wireline tool carrying the north-seeking gyroscope can easily be rotated at the surface to several different toolface (roll angle) positions to determine the bias present on either of the transverse gyroscopes (i.e., along the x and y axis of the tool) when the tool is at the surface. This bias can be used to verify the accuracy or to correct the gyroscope measurements.
In the MWD environment, the above-noted advantages of the wireline systems are not present. The MWD surveys are usually taken during drill pipe connection times during the drilling of the wellbore, which intervals are relatively short—generally one to four minutes. Power in the MWD tools is generated downhole and/or provided by batteries. To conserve the power, it is desirable to switch off the gyroscopes when not in use because the gyroscopes consume considerable power. For MWD tools utilizing turbine-alternator, the power is generated by flow of the drilling fluid (“mud”) which is interrupted at each pipe connection. Even if the power could be applied continuously, the difference in the bias measured at the surface prior to the drilling and post drilling is not considered an accurate measure due to the very long time between drilling assembly trips, which are typically between 20 and 200 hours.
Earlier 2-axis (X-Y) gyro tools could be used for North-Seeking gyrocompass operations when the tool is vertical up to about 60 degrees inclination. This is a static operation, which is done during pipe connections while there is no motion of the drillstring. Gyroscopic steering of oilfield drilling assemblies is typically accomplished by the addition of a 3rd (Z-axis) gyro which is oriented to measure the rotation of the toolface along the long axis of the drillstring. An example of such a device is disclosed in U.S. Pat. No. 6,347,282 and U.S. Pat. No. 6,529,834 to Estes et al, having the same assignee and the contents of which are incorporated herein by reference. With slim (1-¾″ OD) tools, there is very little room to accommodate a 3rd gyro axis mounted crosswise to the X and Y axes, which are often realized in a single, 2-axis rate gyroscope.
Prior art devices have added a smaller, less accurate rate gyro in the Z-axis to allow direct measurements of the angular rotation rate in the Z-axis (toolface). By integrating this Z-axis rate, these tools can track changes in the toolface angular orientation as the drilling motor and deviation device (bent sub) are sliding down the borehole. However, the resultant accuracy leaves a lot to be desired.
Attempts by the applicant to track toolface using only the Rate-X and Rate-Y measurements have been made using a modification of the original gyrocompassing technique. On the theory that there may be some time periods when the BHA is still enough to allow using the conventional North-seeking operation to work, a “Fast Intermittent Gyrocompassing” technique was tested. Laboratory tests showed that the extreme difference between earth rate (15 deg/hr) and toolface changes during typical drilling (˜45 deg/sec or ˜162,000 deg/hr) caused detection problems. There is no guarantee the platform will ever be stable, and no independent indicator of a sufficiently stable condition. Even minuscule drillstring relaxation after a drilling period is likely to introduce large rate errors in trying to measure the earth's rotation.
It is desirable to be able to track toolface changes during steering, using only an (X-Y) 2-axis rate gyro sensor. The present invention satisfies this need.